An optimized control strategy for integrated solar and air-source heat pump water heating system with cascade storage tanks

By integrating non-concentrating solar collectors and air-source heat pumps (ASHP) in parallel, the water heating system with cascade storage tanks shows great potential for energy saving when the control strategy is well designed. In this study, a novel feedback control strategy is proposed for this system, in which the ON/OFF of ASHP connected to the heat storage tank (HST) is activated by comparing the tank temperature to a set of reference temperature curves that are functions of time. The reference temperature curves are established based on the operation scenarios of ASHP at HST, varying with seasonal environmental characteristics and users' thermal demands. To illustrate the advantages and test the robustness of the proposed control strategy, the systems running with the different control strategies are studied for a residential building located in different climate conditions through TRNSYS simulation and experiments. The results show that as compared to the commonly-used thermostatic strategies, the proposed control strategy could keep HST water temperature at a lower level during the daytime, and increase the operation efficiencies of solar collectors and ASHP at HST, which maximize the solar energy harvest. Therefore, the working hours of ASHP are significantly shortened, and thus up to 26% of the annual energy consumption could be reduced. The results also show that the control strategy is more effective in the high irradiation zones. This study is significant because it provides design guidelines for the easy-to-operation control strategy of the integrated parallel solar thermal and ASHP water heating systems. (C) 2020 Elsevier B.V. All rights reserved.